a. Field of the Invention
This invention relates to an electronic image processing device which uses automatic image registration techniques and equipment for assuring valid registration of the images and more particularly to an apparatus in which several regions are used for the comparison.
b. Description of the Prior Art
Many image processing steps require two or more images to be mutually aligned, based on the content of the images. Several methods exist to determine the amount of mis-registration, and to utilize the detected mis-registration to re-register images for further processing. The simplest re-registration technique is to shift the image data of one or more images to align with a single reference image. The amount of shift used is detected by the quantitative mis-registration detection.
Quantitative mis-registration detection methods currently in use include edge recognition, point recognition, correlation of row and column sums, correlation of local regions and others. These methods are useful in many situations, but have limitations due to accuracy, computation complexity, and limited application in certain image situations.
Another quantitative mis-registration detection method performs a comparison of a test image local region with a reference image local region by summing the absolute value of the difference of the two local regions, on a pixel-by-pixel basis. One of the two local regions is then shifted along the X axis by one pixel and the summation performed again. This operation is repeated a number of times and the summation result recorded for each position. The X axis shift amount is then returned to the original point and the region is shifted along the Y axis by one pixel. The X axis shifting and summation is then repeated for this new Y axis position. This Y axis shifting, and subsequent X axis shifting and summing is repeated a number of times. The recorded summation results are then examined and the minimum value noted. The minimum summation result corresponds to the pixel shift that best aligns the features within the local region.
The amount of shifting is determined by the expected maximum mis-registration amount, and is centered about the nominal position. The size and position of the local region is selected to include an image feature which has significant contrast and non-repeating information content in both the horizontal and vertical directions.
This mis-registration detection method is similar in operation to unnormalized correlation of local regions, but requires much simpler computations as no multiplications are needed.